In giving my friends GTD advice I always urge them to go with smaller, but it really depends on how you intend to use it. If you're going to use it for your calendar and lists, then small is best, but if you want to do planning with it, them maybe you would need more room.

A caveat, however, is that you might be able to lay it flat and use two pages as a single surface?

Either way, your tools have to be fun or at least not offer resistance to their use.

GOD:

Totally see what you're saying re portability--right now I use a Levenger Ballistic Pocket Briefcase for Index cards for capture when I'm out and about. And a Filofax A5 would still be portable (it's hardly an anchor--it's 9 1/4" X 7 3/4"). One could also take the lists out of the Filofax if need be (an errands list, for example), if you don't need the whole planner.

But I am really at my desk 90% of the time. I have heard of people having both a larger and smaller Filofax, and moving pages from the smaller to the larger when needed (such captured notes). But I don't want to start with two Filofaxes. :)

Happy Thanksgiving to those who are observing it today!

Sorry to reply so late after your initial post, but I've used Personal and A5 size filofaxes.

For me, a big plus of the A5 filofax is that it can take A4 (standard office paper this side of the pond) folded once- that means I can tuck things I need to remember into the filofax without lots of folding or worry about crumpling them. I can also use pre-printed refills from diyplanner.com without too much faff. And, of course, the A5 gives lots of space for note-taking. The narrow page width of the Personal really crimped my style.

All that said, the A5 is quite a lump to carry around...